1. Field of the Invention
This disclosure is related to the field of release coatings which provide anti-adherence characteristics to rolled liners without introduction of problematic amounts of haze in later lamination.
2. Description of Related Art
Release coatings are generally used to prevent things from sticking together. This simple statement and function encompasses a broad base of technology and a large global industry involving both silicone and non-silicone materials. A very common release coating in the industry utilizes thermal curing based generally on the following reaction:

In the above reaction, a high molecular weight silanol prepolymer (such as α,ω)-dihydroxysilanol of polydimethylsiloxane (PDMS) structure which has a molecular weight of about 5 kg/mol) reacts with a lower molecular weight silane (such as one with a molecular weight of about 2 kg/mol). The high functionality of the silane provides for the crosslinking of the silanol and the resultant curing of the coating through the formation of an infinite 3D polymeric network. The reaction proceeds slowly at room temperature, but dramatically accelerates in the presence of a catalyst and under elevated temperatures. The reaction is dehydrogenative condensation which is accompanied by evolution of dihydrogen.
Silicone release coated films are commonly used as liners in order to protect adhesive layers that are applied to other films or materials (called substrates in this disclosure). In a common embodiment, the liner will comprise a polyethylene terephthalate (PET) film which is then coated with a silicone release agent to form a liner. This liner will be manufactured for later use where the silicone release agent is coated with an adhesive, which adhesive is applied to a substrate and laminated to attach the adhesive to the substrate. Alternatively, a substrate may be coated with an adhesive, which is dried or cured, and then a silicone coated release liner is laminated to the adhesive. In either case, the liner often remains in place for a period of time after lamination to protect the adhesive. During the period where the liner is attached, it is commonly necessary that the combined material (substrate and liner) be generally optically clear and show little optical distortion. This is especially true if it is to be used for later films which are to be used in applications which require very high optical clarity such as touchscreens.
In the manufacturing of such a liner, the liner is generally manufactured in long sheets which are then rolled up to provide for easier shipping and storage. Specifically, a long roll of the film is provided which is unrolled, coated with the release agent during the unrolling, and rerolled to form a liner roll. The problem is that after the liner is rolled, if it sits for a period of time, the adjacent layers will often stick together between successive rolls. The adhesion (sticking together of the silicone release coating to the backside of the adjacent film) can be caused from a number of features, but is generally believed to occur more often when both the film and the release agent surfaces are very smooth and generally occurs more in larger rolls where there is more tension on the wound film. While the specific mechanism of attachment is not totally understood, it is believed that when very smooth surfaces are placed in contact, they tend to adhere due to sufficient surface contact allowing Van Der Waals forces to have macroscopic effect. This is similar to the effect generated by placing a small amount of water between two panes of glass.
Films with reduced smoothness generally don't stick together and therefore one methodology to eliminate the sticking problem is simply to make one or both of the surfaces rough. While this can work in some embodiments, it often makes the resulting film appear hazy. If the film does not need to be optically clear, this is generally not a problem. However, for applications where the film needs to be optically clear it is not an acceptable solution.
The adhesion is problematic because it can result in a number of undesirable alterations to the liner structure. In the first instance, the release layer may adhere to the opposing side (back) of the film layer with greater force than the primary side (front) of the film layer. It, thus, can be removed from the front side during unrolling which can result in the liner having defects present during the adhesive lamination where the release layer has been removed. This can result in the liner not being correctly removable from the adhesive and can result in an undesirable final product.
A more major concern is that the adhesion can be such that the film is torn during unrolling and prior to the adhesive addition, rendering part of the liner roll unusable. In an extreme situation, the adhesion could become so bad that the roll cannot be unrolled at all, destroying the entire liner roll.
Adhesion is particularly a problem for smooth, clear polymer films, especially smooth, clear polymer films such as PET that are coated with a silicon release coating. These films and coatings are often designed to be very smooth to provide for low haze and the adhesion strength is enhanced through an increase in surface area and, thus, surface smoothness of the polymer film sheets. Accordingly, PET films coated with silicone release coatings when rolled for purposes of storage and transportation tend to have an increased affinity to bond to themselves as their quality (transparency) improves.
Generally, the commonly used anti-blocking agents (which are chemical layers) are unsatisfactory for use in these liners. Such anti-blocking agents are commonly applied to the initial film roll (prior to the film being coated with the release layer), but the act of applying the release layer generally results in them being removed or covered. Thus, the anti-blocking agent is often useless to prevent adhesion within the liner roll. Alternatively, chemicals can be mixed into the release layer, but these can produce adverse reactions. Further, the chemicals applied for anti-blocking can be incompatible with the release layer and therefore cannot be used in the liner roll as they can result in damage to the release layer, an increase in haze (a loss of clarity and a reduction of specular reflectance) or other undesirable properties.